I haven't the foggiest idea why onions and asparagus shouldn't be near to each other but........
I bought a fair few Japanese onion sets last autumn intending to plant the lot straight away. My Dad (whose plot it used to be) decided to "help out" and nabbed one of the two bags. He then planted them into several deep seed trays to "get them started".
Sounded a good idea....
I took the other bag, which he hadn't spotted, home and then straight onto the plot.
In due course the seed-trays full of onions began to sprout in my dad's cold greenhouse.
All fine so far......
:blink: Problem came when I needed to move 4 huge seed trays (double size & deep) to the plot a mile and a half away with no car! So....one tray at a time I cycled them down to the plot in my bike trailer (it's big).
The last of the Autumn onion sets went in only recently as I had missed one hiding in a cold-frame. As they had been sown in a peat-based compost replanting wasn't too difficult (although not so easy as sticking them straight in would have been!!).
All of the onions (those planted direct last Autumn and the last lot from the seed-tray) are standing up well and it's hard to tell the difference.
If you lift them from more compact soil you do stand the risk of damaging the roots to some degree.
I think unless you have some more information on why asparagus and onions should not go near each other I'd leave them put.... or experiment and move just a few to see what happens???
I'm still learning from my "mistakes".... sometimes they work out well!